Electrostatic stabilization of colloidal particles is one of the oldest and most common tools to produce stable colloidal suspensions. Yet, despite this ubiquity, attempts to control the phase behavior of colloids by controlling their electrostatic properties have been limited to somewhat coarse techniques: such as dialysis, slow hydrolysis of acid/base precursors, or the absorption of reactive vapors.
Control over the repulsion between the surfaces of like-charged colloidal particles is essential to food technology, pharmaceutical products, paints, mineral refinement, construction materials, etc. This repulsion is idiomatically referred to as electrostatic stabilization, but this shorthand is a misnomer for a force that is essentially due to the entropy of the dissociated counter-ions. As such, this force is modulated both by controlling the dissociation of ionizable moieties of the particles, and by controlling the concentration of free ions in solution. However, the ability to control salt concentration remains a rather blunt instrument.
For example, the state-of-the-art method to produce charged colloidal crystal arrays (CCAs) still appears to be passive, bulk dialysis of the colloid against deionized water, followed by equilibration of the fluid with ion exchange resins. Equilibration of small colloid volumes by diffusion can occur relatively quickly, but equilibration times increase with increasing volume, and the fragility of these crystals precludes adequate mixing. When the structure of the colloidal suspension is not important, electrodialysis presents a far more effective means of deionization; however, the parallel arrangement of diluate and concentrate channels causes persistent electrokinetic currents of the colloidal particles, and the operation of the device depends on the constant production of H2 and O2 gas by electrolysis of water at the electrode surfaces to sustain the currents that drive charge separation.
Microfluidic technology now also offers updated versions of microdialysis tools, whereby co-flow, cross-flow, and hybrid flow (dead-end sample chamber separated from a flushing stream) devices allow for the rapid mixing and equilibration of colloids, together with good control over the compositions of the fluids that are mixed or flushed. Yet, these devices are still limited to small volumes, and integration of fine spatial control over fluid composition would require sophisticated fabrication techniques and the simultaneous control of tens or hundreds of fluid pumps.
There remains a need for method and systems for controlling the salinity of solutions and the surface charging of colloids.